1. Field of the Invention
The present invention relates to a digital locator which is applicable to location fault point of power system.
2. Description of the Prior Art
First, constitution of a conventional locator and the principle of detecting of a fault point using this locator are described below.
FIG. 1 is the simplified block diagram of a power system to which the digital locator related to the invention is applicable. The power system connected with the digital type locator is comprised of the a following; generator 1, bus 2, transmission line 3, fault point 4, current transformer 5, potential transformer 6, digital locator 10, a current transformer 11 which converts the output of the current transformer 5 into an adequate value, a voltage transformer 12 which converts the output of the potential transformer 6 into an adequate value, filtering circuits 21 and 22 which respectively eliminate restricted frequency bands, a circuit 31 which samples AC input current at constant intervals before sequentially A-D converting it into digital values, and operation circuit 100. The reference numeral 4 denotes the fault point in the power system.
In conjunction with means for operating a digital type locator, bus voltage v can be calculated by applying an equation shown below. ##EQU1## where R represents the resistance value of the transmission line up to the position where the fault point is present, L the inductance value, and i is the fault current, respectively. Based on the equation shown above, both of the following equations can be established, which respectively denote two constant time units being different from each other. ##EQU2## It is conventionally known that the resistance value R and the inductance value L can respectively be determined by working out a simultaneous equation derived from the above integrated equations.
When executing operations using any digital type locator, it is necessary for the locator to determine the approximation by applying discrete values produced from sampling of the integrated values at constant timewise intervals. Consequently, the applied approximation generates specific frequency characteristic, which in turn causes error to easily occur. To improve frequency characteristic generated by the approximation, a preceding art in conjunction with Japanese Patent Laid-Open No. 62-40019 (1987) proposes an invention titled by "Digital Distance Relay".
This preceding invention provides means for approximating the integrated value of current by applying the integration shown below, in which current values are denoted to be i(n), i(n+1), and i(n+2) after continuously sampling those current values at constant sampling intervals between times t.sub.n, t.sub.n+1 and t.sub.n+2, respectively, ##EQU3## However, since the integrated value is approximated by applying the integration shown above, error significantly grows in higher frequency bands.